This disclosure relates generally to methods for run-time detection of streaks produced in scanned images.
A main drawback of paper-fed scanners, such as CVT (Constant Velocity Transfer) scanners, is their vulnerability to streak defects. Because the sensors equipped in such scanners are open to the environment, extraneous dirt, such as paper fibers, may cover or partially cover some of the sensor elements and stick there for a short time or as long as scanning several pages. As a result, streak defects, formed as straight thin lines, will appear on the images of scanned documents in various lengths. Some mechanical methods may be used to keep dirt off the sensor surface, but usually are not sufficient to completely eliminate all streaks.
Another method proposes the use redundant sensors for streak detection. For example, two identical sensor arrays can be spatially separated with a certain distance for a black and white (B/W) scanner and streaks can be determined by comparison of the outputs of two arrays. A method to use 4-row sensors to detect streaks for color scanners has been also been proposed in D/A3408. Obviously, methods using redundant sensors require new hardware designs and cost more money. Although it is not too difficult to detect scanner streaks by a post-scanning image processing, it is often not practical due to time or memory limitations. It would be desirable to have a method which does not require major modification of existing scanning mechanisms while providing sufficient run-time streak detection to prevent severe streaks, such as inches-long, dark lines appearing in images of scanned documents.
If a streak defect is detected in a scanned document, the next task is to replace it with “correct data”. Replacing corrupted data involves guessing what the actual data should have been, based on neighboring data. The most common approach is linear interpolation and works very well for streak removal from images with smooth backgrounds. However, if the left and right neighborhoods of a streak have a textured background, such as found in halftone structures, linear interpolation does not provide a satisfactory result. 20031611-US-NP describes a method of run time streak removal that removes streaks from textured backgrounds with satisfactory results.